I started a thread a while back on this and spoke at length with a fox expert on the phone. His first suggestion was to get or borrow a dog and let it do its thing in the garden. Foxes don't like dog scent as they see dogs as dominant. If that's not possible, he suggested getting some clumps of uncut dog hair and placing them around the garden in socks or nets. His final suggestion was the 'water scarecrow' which detects movement in the garden and sprays water in any or every direction which foxes don't like; problem with these is that the motor is fairly noisy for night time.
In my case, i followed his instructions to find out if there was a den being built under my shed. It turned out that there wasn't so i didn't bother going ahead with using any deterrants.